Printing plates using an ink repellent layer such as a silicone rubber layer or a fluorine resin layer without using any dampening water, especially photosensitive planographic printing plates obtained by selective exposure and development have been variously proposed.
The waterless printing refers to a printing method which allows printing without using any dampening water by letting the image area and the non-image area exist basically on almost the same plane, with an ink receiving layer formed on the image area and an ink repellent layer formed on the non-image area for allowing the ink to be formed on the image area only by using the difference of both the areas in ink receptability, and by transferring the ink formed on the image area onto a printing medium such as paper. Usually the non-image area is formed by an ink repellent substance such as silicone rubber or fluorine resin, etc.
For example, proposed positive precursors for waterless planographic printing plates include a precursor of waterless planographic printing plates with a photopolymerizable photosensitive layer and a silicone rubber layer as an ink repellent layer formed on a substrate and a precursor of waterless planographic printing plates with a photodimerizable photosensitive layer and a silicone rubber as an ink repellent layer laminated on a substrate. As a negative precursor of waterless printing plates, JP-B-61-54222, etc. propose a precursor of waterless planographic printing plates in which a photoreleasable photosensitive layer obtained by crosslinking a partial esterification product of 1,2-naphthoquinone-2-diazido-5-sulfonic acid chloride and phenol formaldehyde novolak resin by a polyfunctional isocyanate is laminated on a substrate, with a silicone rubber layer as an ink repellent layer laminated further on it. A precursor of waterless planographic printing plates in which a fluorine resin using 1H,1H,2H,2H-heptodecafluorodecyl methacrylate is used as an ink repellent layer instead of a silicone rubber layer is also known.
Any of these precursors for waterless printing plates is exposed to active light through a positive film when the precursor is positive, or through a negative film when the precursor is negative. In the subsequent development treatment, the ink repellent layer is removed only at the area corresponding to the image area to expose the photosensitive layer or the substrate below it as the case may be, for forming the image area where ink can be formed. Usually in the case of a positive precursor of waterless planographic printing plates, the ink repellent layer is removed at the non-exposed area, to form the image area where ink can be formed, and in the case of a negative precursor of waterless planographic printing plates, the ink repellent layer is removed at the exposed area, to form the image area.
Among the above precursors for waterless planographic printing plates, as a photoreactive compound used in the photosensitive layer of a precursor of waterless planographic printing plates, for example, JP-B-61-616, etc. disclose a phenol formaldehyde novolak resin having quinonediazide groups. As another photoreactive compound, for example, an acetone pyrogallol resin having quinonediazide groups, etc. is also preferably used. However, all of quinonediazide groups, phenol formaldehyde novolak resin and acetone pyrogallol resin are structurally rigid, and make the entire photosensitive layer rigid. So in mass printing, it happens that the photosensitive layer is simply destroyed, and as a result, that in the non-image area, the photosensitive layer falls off together with the ink repellent layer formed on it. If the ink repellent layer falls off, printing cannot be continued further. For this reason, the conventional negative precursors for waterless planographic printing plates are said to have a problem in printing durability.
To improve the printing durability, various studies have been made. For example, JP-A-8-69110 proposes a waterless planographic printing plate using a compound with quinoneazide added to a low molecular compound having a specific structure for making the quinonediazide compound itself flexible in the photosensitive layer, but the compound has a problem that the film formability is not sufficient.
JP-A-8-76368 proposes a waterless planographic printing plate using a photosensitive layer containing a quinonediazide compound and a binder resin such as a polyurethane for making the photosensitive layer flexible. However, the mixture consisting of a quinonediazide compound and a polyurethane is poor in compatibility, being likely to form an islands-in-sea structure, though the waterless planographic printing plate obtained is excellent in printing durability since the photosensitive layer is flexible. If the photosensitive layer has an islands-in-sea structure, each halftone is deformed though halftone dots can be reproduced, and it is difficult to obtain the intended color tone in the prints obtained.
JP-A-7-43895 and U.S. Pat. No. 5,631,119 propose an image forming material, the photosensitive layer of which contains an o-quinonediazide compound and an additive capable of reacting with the photoreaction product of the o-quinonediazide compound by heating to form an alkali insoluble substance, and has a water soluble high polymer layer or furthermore a silicone rubber layer formed on it in this order. This image forming material is exposed on the entire surface to active light, heated or irradiated using a thermal head or laser beam in the form of an image, and developed using an alkaline developer to remove the silicone rubber layer in the non-heated area or the non-irradiated area as well as the photosensitive layer. In this method, since the entire photosensitive layer is removed together with the silicone rubber layer, image reproducibility is low. Furthermore since the halftone cells are deep, it is difficult to fill the entire cells with the printing ink, and especially the image reproduction at the highlighted area becomes disadvantageously difficult.
The present invention overcomes the disadvantages of these prior arts by providing a precursor of waterless printing plates, which is remarkably improved in both image reproducibility and printing durability without being lowered in developability, printability, solvent resistance, etc., by letting the photosensitive layer contain a polyurethane having quinonediazide groups, and which can be used for both the method of obtaining a waterless planographic printing plate by ordinary exposure and the method of obtaining a waterless planographic printing plate by laser irradiation, by letting the photosensitive layer further contain a light-heat converting material.